1. Field of the Invention
This invention relates to a flux for the submerged arc welding of steel, which satisfies the requirement that the weld metal should possess low oxygen, low nitrogen, and low hydrogen content, and materializes results in welding of steel with a weld bead of good shape and with no weld defects.
2. Description of Prior Art
For improvement of the toughness of weld metal, reduction of the oxygen content of the weld metal is one of the effective measures. In welding methods such as the submerged arc welding method which necessitates use of flux, increase of the basicity of the flux brings about an effective decrease in the oxygen content of the weld metal. With a view to enhancing the toughness of the weld metal, therefore, efforts have been made to heighten the basicity of the welding flux.
The basicity of the flux cannot be increased without entailing the various problems indicated below. With the conventional commercial fluxes, the lowest possible oxygen content of the weld metal that can be obtained at all is about 300 ppm.
The first problem is that as the basicity of flux is increased, the regulation of physical properties of flux becomes difficult, so that the form of a weld bead degrades and the weld defects such as slag inclusion and undercut occur.
The second problem is that in the case of a fused type flux which is produced by fusing raw materials for the flux as in an electric furnace, discharging the fused blend from the furnace, cooling and pulverizing it into particles of a suitable size, and drying, the flux during the course of production absorbs water such as the moisture in the ambient air, causes an increase in the amount of diffusible hydrogen in the weld metal, which induces cold cracking and pitting in the weld metal. Fluxes generally absorb water such as the moisture in the ambient air in their fused state. The ability of the fused type flux to absorb water (capacity for saturation with water) is lowest when the flux is in its neutral state, and it increases as the basicity decreases or increases from the neutrality as shown in FIG. 1. When the basicity is on the side higher than the neutrality, amount of water absorbed during the course of flux production increases in proportion to the degree of basicity. In the case of the fused type flux, therefore, the content of diffusible hydrogen in the weld metal increases in proportion as the basicity of the flux is increased for the purpose of lowering the oxygen content of the weld metal, exposing the weld metal all the more to the possibility of sustaining cold cracking and pitting.
The third problem is that when fluxes of various composition are used for welding, the nitrogen content of the weld metal increases in proportion as the oxygen content thereof decreases. One possible cause for this phenomenon is that the reaction of C+O.fwdarw.CO.uparw. in the molten pool decreases in consequence of the decrease in the oxygen content and the shielding action of CO serving to preclude the inclusion of ambient air (consequently nitrogen) is lowered. Another theory is that oxygen affects the mechanism of nitrogen absorption in the gas-metal boundary and the molten slag-metal boundary.
Nitrogen is known to be an element which impairs the toughness of the weld metal. The fact that the nitrogen content is increased by the measure taken to lower the oxygen content spoils the fundamental object of the measure taken to lower the oxygen content.